<!DOCTYPE>
<html>
  <head>
    <meta charset="utf-8">
    <title>IFE JavaScript Task 01</title>
    <script defer src="task.js"></script>
    <style>
      div{
        border:solid 1px #333333;
        padding:30px 10px;
        flex-grow:1;
        display: flex;
        margin: 0 10px;
      }
      .container{
        width: 800px;
        margin: 100px auto 0 auto;
      }
      button{
        flex-grow:1;
        margin: 20px;
        padding:10px 0 ;
      }
      .btnC{
        border:none;
        margin-top: 0;
      }
    </style>
  </head>
<body>
 
 <div  class="container">
   <div>
      <div>
        <div></div>
        <div></div>
      </div>
      <div>
        <div></div>
        <div></div>
      </div>
   </div>
   <div >
      <div>
        <div></div>
        <div></div> 
      </div> 
      <div>
        <div></div>
        <div></div>
      </div>
   </div>
 </div>
 <div class="container btnC">
   <button>前序</button>
   <button>中序</button>
   <button>后序</button>
 </div>
</body>
</html>